Method and system for transmitting a data frame of an electromagnetic telemetry signal to or from a downhole location

A method for transmitting at least a portion of a data frame of an electromagnetic (EM) telemetry signal from a downhole location, the method being performed in an electronics subassembly communicative with an EM carrier frequency signal generator and comprising a digital encoder having a processor and a memory containing encoder program code executable by the processor, the method comprising:

• (a) encoding, using the digital encoder, at least a portion of a long header into a header waveform having a first frequency, wherein the EM telemetry signal comprises the long header and the at least a portion of the data frame, and wherein the long header is independent of a content of the data frame, and is used to synchronize communications between an EM signal transmitter and an EM signal receiver;
• and
• (b) encoding, using the digital encoder, the at least a portion of the data frame and any remaining portion of the long header into a data waveform having one or more additional frequencies different from each other and from the first frequency, wherein the first frequency and the one or more additional frequencies are sufficiently separated to permit demodulation of the at least a portion of the long header encoded into the header waveform and the at least a portion of the data frame and any remaining portion of the long header encoded into the data waveform;
• (c) digitally combining, using the digital encoder, the header and data waveforms into a combined EM carrier waveform;
• and
• (d) generating, using the EM carrier frequency signal generator, the combined EM carrier waveform across an electrically isolated gap of the gap sub assembly;
• and
• (f) transmitting the combined EM carrier waveform.